Duckling safety with wood treatments

I am a new duckling daddy of now 6 week old ducklings. A Blue swede and Rouen. They have been living inside a very large cardboard box in our house here in California while I build their duck castle outside...

I am limited with my energy, due to Chronic Lymes disease, so building a seriously safe predator proof pen, has taken longer than hoped for. Would the following temporary situation be safe for them? :

Allow them to roam around on my 12" x 16" redwood deck that is enclosed by stucco on 1 side and 20 year old redwood fencing on the other three side? If so? Would it be safe to stain or seal to help maintain decking? Of course they would be locked in a house at night...

I also have many other safety questions, but thought I would just start with that